The invention relates to a method and a device for printing with error or fault correction. More particularly, it relates to a print head for an ink-jet printing system having an error or fault correction device, and to a method of correcting errors or faults when printing with an ink-jet printing system.
In ink-jet printing systems, tiny droplets of ink are produced by one or more print heads and are applied to a printing material. In order to produce the droplets of ink, the print head is formed with a multiplicity of nozzle openings, each of which is connected to a respective pump mechanism. Pump chambers of the pump mechanism are filled with printing ink, and a device is provided by which positive or excess pressure can be produced briefly in the pump chamber. Due to the positive pressure, a small quantity, respectively, of printing ink is displaced and escapes via the nozzle opening. By a linear configuration of a multiplicity of nozzles for the same color, it is possible to print a large region of a page and an entire page, respectively, with a printing ink. If different inks, for example cyan, magenta, yellow and black, are then disposed after one another, it is possible to produce over an entire page a colored print resulting from the overprinting of a color separation for each printing ink. In practice, however, it has repeatedly been shown that, during the operation of the print heads, individual nozzles can fail and, thereby, a respective colored point or location is missing on the printing material. These missing print points can be detected by the eye and, to some extent, considerably disrupt the printed image.
In order to monitor the nozzle for the serviceability thereof, and to correct nozzles that have failed, it has been proposed, for example in U.S. Pat. No. 6,270,187, to monitor the serviceability of the individual nozzles continuously. As long as no fault results from this monitoring, the printing material would be printed in accordance with the stipulations. However, if it is detected that one of the nozzles has failed, it is proposed that all the nozzles in the row of colors which lie between the nonfunctioning nozzle and the end of the rows of nozzles be switched off. The task of the nozzles which have been switched off is then taken over by the other nozzles which are still functioning. In the most undesirable case, however, when a nozzle lying in the center of the row of nozzles should fail, only half of the row of nozzles can still be used, and therefore the printing speed is reduced considerably. As an alternative thereto, it is proposed that the printed point of the failed nozzle be set by an adjacent nozzle element, or that a substitute row of nozzles be provided, which is always then applied when a nozzle element fails.
In U.S. Pat. No. 5,581,284, it is proposed that, upon the failure of a nozzle, the colored point of the nonfunctioning nozzle be replaced by a colored point of a different color. Although this method can avoid excessively high visibility of the failure of a single nozzle as a result of a colored point not being set on the printing material, the disadvantage is that this method necessitates another, highly differing color (for example magenta) being set instead of the correct color (for example cyan). This type of correction is also visible to the human eye, in particular when a contribution to a colored line or colored area is to be printed with the failed nozzle.
It is accordingly an object of the invention to provide a method and a device for printing with error or fault correction, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type and which provide a print head having a correction mechanism, and a method of correcting a print in the event of failure of a nozzle, wherein the correction is barely visible to the eye and the printing speed is simultaneously maintained.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a printing device for an ink-jet printing system, comprising at least one row of nozzles for overprinting a first process color, and a row of nozzles for overprinting a correction color. The correction color is different from the process color.
In accordance with another feature of the invention, in addition to the one row of nozzles for overprinting the first process color, further rows of nozzles for overprinting further process colors are provided. The process colors overprintable by the first and the further rows of nozzles are cyan, magenta, yellow and black.
In accordance with a further feature of the invention, the printing device further includes a device for monitoring serviceability of the nozzles for overprinting the colors.
In accordance with an added feature of the invention, the device for monitoring serviceability is a device for monitoring voltage variation when controlling the nozzles for overprinting the colors.
In accordance with an additional feature of the invention, the printing device further includes an inspection system disposed between the rows of nozzles for overprinting the process colors, on the one hand, and the row of nozzles for overprinting the correction color, on the other hand, for monitoring the printed image applied by the process colors.
In accordance with yet another feature of the invention, the printing device further includes a device for controlling each individual nozzle of the row of correction nozzles when a malfunction of the nozzles for overprinting the process colors is detected.
In accordance with yet a further feature of the invention, the printing device further includes an inspection system disposed between the rows of nozzles for overprinting the process colors, on the one hand, and the row of nozzles for overprinting the correction color, on the other hand, for monitoring the printed image applied by the process colors. The device for controlling the individual correction nozzles is automatically activatable upon detection by the inspection device of a malfunction of the process nozzles.
In accordance with yet an added feature of the invention, the device for controlling the individual correction nozzles is automatically activatable upon detection of a malfunction of the process nozzles.
In accordance with yet an additional feature of the invention, the nozzle-controlling device serves for manually activating the individual nozzles of the row of correction nozzles.
In accordance with still another feature of the invention, the correction nozzles are connected to a color reservoir having a correction color formed of a weighted sum of the process colors.
In accordance with still a further feature of the invention, the correction color is a gray tone.
With the objects of the invention in view, there is also provided a method of correcting failure of a nozzle for overprinting a process color in a print head of an ink-jet nozzle system having a plurality of rows of nozzles, to each of which a respective process color is assigned. The method comprises detecting a failure of a nozzle for overprinting one of the process colors, and depositing a correction color different from the color of each of the process colors at a fault location on the printing material, which is caused by the failure of the nozzle.
In accordance with another mode, the method of the invention further includes monitoring each individual nozzle for overprinting the process colors for detecting the failure of the nozzle for one of the process colors.
In accordance with a further mode, the method of the invention further includes detecting the nozzle failure by an optical inspection system for monitoring the printed image applied to the printing material.
In accordance with an added mode, the method of the invention further includes activating a nozzle from a row of nozzles of a correction head for applying the correction color.
In accordance with an additional mode, the method of the invention further includes forming the applied correction color of a weighted mixture of the process colors being used.
In accordance with a concomitant mode, the method of the invention further includes forming the applied correction color as a gray tone.
According to the invention, in order to correct the failure of a nozzle in a print head, the point or location at which a nozzle has failed is thus occupied by a correction color. As a result of the use of a row of nozzles for overprinting a correction color, a row of nozzles is thus available for the entire width of the print head, and by the aid of the row of nozzles, each individual nozzle of the print head can be corrected. Since the correction color is different from each of the process colors which are used, the correction color can be used for any of the process colors being used. The corrected printing image will certainly not correspond exactly to the originally intended printed image, but it is possible to correct the printed image in such a way that the failure of the nozzle cannot be detected or can be detected only with difficulty.
The process colors cyan, magenta, yellow and black are normally used in one or more print heads. The correction color that is selected is therefore advantageously a gray, which results from a suitable percentage mixture ratio of the indicated process colors. In order to detect whether a correction nozzle must be activated, in principle all those methods are available by the aid of which it is possible to detect whether one of the nozzles of the process colors has failed. In particular, each individual nozzle of the process colors can be monitored for serviceability thereof during the overprinting. When it is detected that a nozzle has failed, the corresponding correction nozzle of the correction head can be activated.
As an alternative thereto, it is also possible to monitor the printed image applied with the aid of the process colors. This is normally done with the aid of a scanner, which is disposed to follow the process color nozzles. The scanner registers the printed image applied by the process print heads and compares it with a desired or nominal printed image. If differences result from this desired-actual comparison and permit a conclusion to be drawn regarding the failure of an individual nozzle in one of the process colors, then the corresponding correction nozzle is activated and the correction color is introduced into the fault location. With the use of the correction head according to the invention, it is therefore possible in the ink-jet method to correct the failure of an individual nozzle regardless of the type of process color.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and a device for printing with fault or error correction, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.